The immunobiological mechanisms associated with acute and chronic GVHD in the clinical setting are not completely understood. Further, the advent of effective immunosuppressive drugs such as cyclosporine while preventing acute GVHD after allogeneic bone marrow transplantation was found to exacerbate the immune dysregulation of self nonself discrimination during immune reconstitution. The failure to develop self tolerance leads to an autoaggression syndrome resembling chronic GVHD. This subproject primarily deals with the immunobiological mechanisms of GVHD studied with the use of in vitro and ex vivo models. The majority of the studies parallel the animal experiments described in project 9 in an attempt to correlate the principles defined in the animal model. On of the principle goals of this subproject is to evaluate the Vbeta region gene repertoire of the T cell receptor of effector cells associated with acute, chronic and syngeneic/autologous GVHD. The polymerase chain reaction will be utilized to identify V region TCR receptor genes of effector cells mediating GVHD in the predictive skin explant assay and attempt to determine is similar V region genes can be detected in biopsies of those patients with acute GVHD. Such studies will determine if GVHD is an oligoclonal disease and will provide the basis for the development of new immunosuppressive strategies (anti V region TCR antibodies). Further TCR V regions gene usage in cells from autologous MCR (donor x donor and from skin biopsic of patients developing chronic GVHD will be compared to determine if there is an autoimmune component in chronic GVHD. In addition, the skin explant assay will be utilized to assess effector mechanisms identifying the role of cytolytic T cell effector mechanisms including natural killer cells. Keratinocyte cultures will be established to assess the role of the keratinocytes as a target cell for GVHD effector mechanisms. The skin explant assay and response to individual components (i.e. keratinocytes, etc.) also will be used to assess the mechanism of action of new immunosuppressive approaches which may downregulate leukocyte adhesion molecules will be further explored. Together, the proposed studies will provide an in depth insight into the immunobiology of GVHD in clinical bone marrow transplantation.